Synching issues?

Since I dont understand the sync feature completely i would like to ask some questions.

-First off, upon connecting the ipod what is it synching? When i hook the usb cable itunes boots up automaticly and it displays syncing users ipod. If i havent changed anything what's the use of synching? BTW i worry that it may delete something so i dont want it to synch without permission.

-Can you synch items from the ipod to the itunes? Because i use different computers from different locations and there seems to be an issue. For example today i wanted to install an app i downloaded from itunes, and since i have some other already installed from my laptop, i get the messaged that the apps not installed on the current itunes will be deleted from the ipod. So i'm hesitant to install this new app, since it is going to delete my existing ones.